Load-bearing axial bearings are used when axial thrust forces are applied to rotors which rotate at high speed. In the case of exhaust-gas turbochargers by way of example, hydrodynamic axial bearings are used to absorb high axial forces resulting from the flow, and to guide the turbine shaft in the axial direction. To compensate for incline positions and the wear behavior in applications such as these, free-floating disks, so-called floating disks, can be used between a bearing comb, which rotates at the shaft rotation speed, and a bearing housing, which does not rotate, in hydrodynamic axial bearings.
Examples relating to this can be found, inter alia, in GB1095999, EP0840027, EP1199486 and EP1644647, the disclosures of which are all incorporated herein by reference in their entireties. The floating disk is radially guided either on the rotating body, that is to say on the shaft or on the bearing comb by a radial bearing which is integrated in the floating disk, for example as disclosed in EP0840027, or on a stationary bearing collar, which concentrically surrounds the rotating body, for example as disclosed in EP1199486. A hydrodynamic axial bearing such as this can be lubricated by lubricating oil from a dedicated lubricating-oil system or, in the case of exhaust-gas turbochargers, via the lubricating-oil system of an internal combustion engine which is connected to the exhaust-gas turbocharger.
During operation, a load-bearing lubricating film is formed between the floating disk, which rotates at only about half the shaft rotation speed, and the shaft or the bearing comb which is arranged on the shaft. Profiled annular surfaces can be provided for this purpose on both sides of the floating disk and form the lubricating gap in each case together with one smooth sliding surface. The profiled annular surfaces include wedge surfaces which are oriented at least in the circumferential direction and which, together with the smooth sliding surfaces, form a converging gap. If sufficient lubricant is drawn into this converging gap, the load-bearing lubricating film is formed. The lubricant propagates in the radial direction because of the effect of the centrifugal force from the floating disk, which is rotating at high speed.
The friction moments on the axial and radial sliding surfaces of the floating disk can influence the rotation speed of the floating disk. At high shaft rotation speeds (e.g., less than 50% of the shaft rotating speed), the floating disk rotates less than half as fast as the shaft. This can result in different relative speeds in the two axial lubricating gaps. The relative speed of the shaft with respect to the floating disk is in this case greater than the relative speed of the floating disk with respect to the bearing housing.
The gap heights which occur in the two axial lubricating gaps are of different magnitude, because of the different relative speeds and the different centrifugal-force effects. Because the bearing size is designed for the smallest lubricating gap that in each case occurs, one bearing gap is overdesigned, and this can lead to an unnecessarily high power loss and to an unnecessarily high oil throughput.